That's because, with the new tax law, Republicans have made important changes to 529 plans that will affect millions of taxpayers, not just the ones saving for college. Before that news, though, a quick primer.

A 529 plan lets families save money for college. Think of it as a love child, born in the mid '90s to your federal and state governments. And they named it, in a flash of creativity, after its relevant section in the Internal Revenue Code.

States generally manage the plans, while the Feds let the money grow long-term, tax-free. Thirty-three states also try to encourage savers with a little short-term reward (or not so little, in some cases): When families in those states make a contribution, they get a deduction or credit on their state income taxes, too.

"That lets people know, 'Look, this is a tax advantage that you can unwrap for yourself right now and be a gateway to additional tax advantages later on,' " says Troy Montigney, who oversees Indiana's 529 program. His state offers families a $1,000 tax credit for contributions.

But that credit means less tax revenue coming in. It's a trade-off for states; they figure it's worth the lost revenue if a tax break gets more people saving for college.

Now, it's these state-based tax breaks that are driving real concern among state leaders about Washington's recent tax overhaul.

What has changed

After Congress' rewrite of the tax code, parents can now use 529 plans to cover tuition not only at colleges and universities, but also at private elementary and high schools. That's a big, sudden expansion, and it has some experts worried.

"This change allows private school families to put their money through 529 accounts and avoid state income taxes," says Nat Malkus, who studies education policy at the American Enterprise Institute, a conservative-leaning think tank. "It is a mess, no matter how you slice it. It's a change from the federal level that puts a number of states in a pretty tough position moving forward."

That's because, Malkus says, if this move entices lots of new families to sign up and many current families to contribute more, then states could end up losing a lot more money to tax breaks.

"I think it would immediately create an unintended hit to the state's budget," agrees Greg Berck of the New York State Council of School Superintendents. "States plan ahead, sometimes multiple years ahead, and New York state will be required to provide a state tax deduction [to parents of students in K-12 private schools] unless the legislature acts to amend our state law."

NPR spoke with representatives from half-a-dozen states that offer a tax credit or deduction for 529 contributions. While some were more worried than others about the potential budgetary hit, all expressed frustration that the expansion came top-down from Washington, giving them no time to plan or budget for it.

"This will have a huge impact on state-run 529 plans. And for states that offer a tax deduction, a major impact on state tax receipts," says Michael Frerichs, who is Illinois' state treasurer.

Frerichs says 529s were built on the idea of patience — of parents slowly saving for college — and that using them to pay for kindergarten is a significant change.

"If [families are] putting money in one month and taking it out the next, they don't really have that advantage of long-term investing," Frerichs says. "And it's really just using them to get around state taxes."

To get around state taxes. That raises a natural question...

Whom does this help?

Most Americans send their kids to public schools. And there's little concern that this change will drive many of them into private schools, since using a 529 to save for the early grades just doesn't make a lot of sense. For early grades, the funds simply don't have enough time to grow — except for high-income savers who can afford to set aside a lot of money from the get-go.

The real benefit, according to the state experts and independent economists NPR interviewed for this story, is for affluent families — many of whom already have kids in private K-12 schools. They can now use their old 529, or open a new one, to help pay that tuition, all while getting a nice state tax break.

Troy Montigney of Indiana says he's hearing from a lot of curious parents.

"We're already fielding — I'll just be honest — a tremendous amount of calls on a daily basis," Montigney says. "About, 'Can I, you know, take a withdrawal right now to pay for a K-12 tuition expense?' "

The challenge for state 529 managers, treasurers and lawmakers is they've had just days to come up with answers. And it's difficult to know how much this expansion of 529s' uses will actually expand the pool of people who use them.

Nat Malkus, at AEI, believes states will take a financial hit and will have to make some tough choices.

"They're either going to have to accept a loss in their income tax base or do something unpopular to repair the hole," Malkus says.

Lots of states, from Colorado to South Carolina, Michigan to Mississippi, could end up feeling the pinch. One of the few states that won't, though, is Texas, where the idea began with Republican Sen. Ted Cruz.

Texas doesn't give families an extra break on their income taxes when they contribute to a 529 because Texas doesn't have an income tax.

Copyright 2018 NPR. To see more, visit http://www.npr.org/.

ARI SHAPIRO, HOST:

Now we're going to hear about a small piece of the new tax law that could have a big impact. It has to do with 529 plans, which let families save for college. The new tax law expands 529s so that parents will be able to use them to cover tuition at private elementary and high schools, too. NPR's Cory Turner reports that the move caught states off guard and could cost them dearly.

CORY TURNER, BYLINE: Think of the 529 plan as a love child born in the mid-'90s to your federal and state governments. They named it in a flash of creativity after a section of the tax code. States generally manage the investment plans while the feds let the money grow long-term tax-free. Many states also try to encourage savers with a little short-term reward. When families make a contribution, they get a credit or deduction on their state income taxes.

TROY MONTIGNEY: Most often, the state tax credit or deduction is the foot in the door.

TURNER: Troy Montigney oversees Indiana's 529 program, where savers get a tax credit. Thirty-three states do this, putting extra skin in the game. That credit in Indiana or deduction in Oklahoma means less tax revenue coming in. States just figure it's worth it if it gets more people to college. But all of this is why Congress' sudden expansion of the program into K-12 schools has some experts worried.

NAT MALKUS: This change allows private school families to put their money through 529 accounts and avoid state income taxes.

MALKUS: It's a change from the federal level that puts a number of states in a pretty tough position moving forward.

TURNER: Malkus says if lots of new families sign up and current families contribute more, well, then states could end up losing a lot more money in tax breaks.

GREG BERCK: I think it would immediately create a unintended budgetary hit to the states' budget.

TURNER: Greg Berck of the New York State Council of School Superintendents says this expansion surprised states. After all, it came top down from Washington, and states haven't budgeted for it. New York offers savers a $10,000 deduction on their state taxes. In Illinois, where Michael Frerichs is treasurer, it's even more generous - $20,000. He says 529s were meant to save long-term for college, and that letting families use them for kindergarten changes things.

MICHAEL FRERICHS: If they're putting money in one month and taking it out the next, they don't really have that advantage of long-term investing. And it's really just using them to get around state taxes.

TURNER: This point is key. Most Americans send their kids to public schools, and there's little concern that this is going to change that because using a 529 for the early grades, it just doesn't make a lot of sense. Literally, there isn't time for the money to grow. The real benefit, according to the state experts and economists I spoke with, is for affluent families, many of whom already have kids in private K-12 schools. They can now use their old 529 or open a new one to pay that tuition bill and get a nice state tax break. Troy Montigney of Indiana says he's hearing from a lot of curious parents.

MONTIGNEY: We're already fielding - and I'll just be honest - a tremendous amount of calls on a daily basis about can I, you know, take a withdrawal right now to pay for a K through 12 tuition expense?

TURNER: The challenge for state leaders - they've had just days to get their heads around all of this, and it's hard to know how much this expansion of 529's uses will actually expand the pool of people who use them. Nat Malkus at AEI believes states will take a hit and have to make some tough choices.

MALKUS: They're either going to have to accept a loss in their income tax base or do something unpopular to repair the hole.

TURNER: Lots of states could feel the pinch. One that won't is Texas, where the idea began with Republican Senator Ted Cruz. Texas doesn't give savers an income tax break because Texas doesn't have an income tax. Cory Turner, NPR News, Washington.

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